No. of parallel BJT? Rule of thumb? - diyAudio
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Old 26th April 2004, 10:41 PM   #1
sam9 is offline sam9  United States
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Default No. of parallel BJT? Rule of thumb?

I can use an SOA graph from a datasheet and derate it but I'm curious about general practice here. So my question is: as a generalized rule how many paralleled pairs of output BJT's per X-watts do people here use. I'm assuming TO-247s with SOAs similar to 2sc5200/2sa1943 or TO-3's such as MJ15024/5.
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Old 26th April 2004, 10:54 PM   #2
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When I was looking at a desing a while back (still in progress, but going slowly) kind of like a beefed up P3A from ESP, I was looking at two pairs of MJL4281a / MJL4302a to output around 250W into a four ohm load. However, I think this is about right on the edg of what I would like to put through them before I would be woried about them going bang, and if I wanted more curent out, I think I would definately add another pair. So, in this case, for these particular devices, I would state a heathly maximum of about 4A per device as my limit.
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Old 26th April 2004, 10:57 PM   #3
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I put a safety factor of two: if the peak current is 2A, I derate the device at 80c and then make sure it can handle at least 4A per SOA chart.
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Old 26th April 2004, 11:06 PM   #4
subwo1 is offline subwo1  United States
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Someone else may also like to touch upon how the power rail voltage also has an impact in determining the SOA location of the output device operation. So if you run more speakers off of a lower voltage-run amp, you may be able to get by with a little less beefiness in the output stage.
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Old 26th April 2004, 11:48 PM   #5
sam9 is offline sam9  United States
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It sounds like I've tended to be more conservative than needed. Partly, this may be influenced by an "ultimate project" I've had in mind for bi-amping a pair of Magnepans which are both low inpredance and low sensativity. My projects to this point had been aimed at getting experience to use in that project. So far I've tended to use one pair per each 75W of nominal output at 8 ohms. I've been able to do this with a 70 degC thermal switch on the device that has yet to trip in normal use. It would appear that I'm running thing cooler than is really necessary. However, I have in mind the beta-droop issue with low impedance loads so I've additional motivations for using more devices than necessary.
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